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Introduction
A glycogen debranching enzyme (GDE) is an enzyme associated with the breakdown of glycogen. The majority of debranching enzymes belong to the GH13 (glycoside hydrolase 13) family, and may further be separated according to functional properties. For example, the mammalian and yeast GDE exhibits both α-1,6-glucosidase and α-1,4-transferase activity, and thus catalyzes two successive reactions in the transfer of glycogen branches. In bacteria and plants, however, the debranching of glycogen is carried out by different enzymes, which possess either α-1,6-glucosidase activity or α-1,4-transferase activity, but not both. For example, isoamylases and pullulanases carry out the α-1,6-glycosidic bond hydrolyzing activity, while glucosyltransferases carry out the α-1,4-transferase activity.

TreX
TreX is an archaeal GDE from the species, Sulfolobus solfataricus. Interestingly, TreX exhibits 74% sequence similarity to the isoamylase from Sulfolobus acidocaldarium, yet TreX itself reveals both α-1,6-glucosidase and α-1,4-transferase activity. Although TreX exhibits this bifunctional activity, its catalytic region differs greatly from other glycogen debranching enzymes. For example, mammalian and yeast GDEs have distinct catalytic sites for the α-1,6-glucosidase activity and α-1,4-transferase activity. These sites are even located at different regions of the polypeptide. In TreX, however, both enzymatic rections take place within the same catalytic region.

Structure and Function


TreX functions to debranch the side chains of glycogen into maltodextrin, and subsequently TreY and TreZ convert the maltodextrin into trehalose. The α-1,4-transferase activity of TreX is responsible for catalyzing the transfer of glucose residues from one 1,4-α-D-glucan branch to another, while the α-1,6-glucosidase activity is responsible for cleaving the lone glucose involved in an α-1,6-glycosidic linkage.

TreX is an oligomer, as it exists in a dimeric state and a tetrameric state, both of which exhibit different enzymatic activities. All subunits are identical, where the monomer contains a total of 612 amino acids. The polypeptide folds into two secondary structures, a β-sandwhich in the N terminal region, comprised of six β-strands and a (β/α)8 – barrel motif in the central domain, comprised of eight parallel α-strands which encircle eight parallel β-strands. The sequence composition of the TreX monomer exhibits a high degree of homology to the isoamylase debranching enzyme of Pseudomona, however the TreX monomer mainly deviates from this similarity in its substrate binding groove and the absence of a calcium ion ligand.

In the dimeric form, the individual subunits are adjacent to each other, where both of the active sites face the same side. In the tetrameric form, two of the associated dimers face each other so as to position the substrate binding sites to the inside of the tetramer, facing each other with a slight offset. Unlike other GDEs, TreX portrays different enzymatic activities in correlation to its oligomeric state. The dimeric form is predominantly associated with α-1,6-glucosidase, whereas the tetratmeic form is associated with α-1,4-transferase activity.

The active site of the TreX dimer contains three important catalytic residues (Asp 363, Glu 399, and Asp 471) situated at the bottom of the active site cleft. It also exhibits a buried interface of 1523 Å, and a total of 6 subsites, where subsite 1 contains the NYWDYDP motif which facilitates substrate interactions of the glucose rings. One of the interesting features which separate TreX from similar isoamylases and pullulanases, it the presence of a helix α4 loop (aa 228-238) situated at the bottom of the substrate binding groove. It is suggested that this helix α4 loop may provide a stable binding region for branched substrates with long chains, and therefore increase the activity of this GDE.

Upon tetramerization, TreX’s active site undergoes a substantial change in conformation, and thus displays both structural and functional differences when compared to the TreX dimer and even to other GDEs. Studies have revealed that the TreX tetramer displays a 4-fold increase in catalytic activity, when compared to the dimer. It is postulated that the conformational change in the active site only occurs in the presence of branched substrates with long chains, such as glycogen, because it establishes an ideal binding site for these types of substrates. To illustrate, the TreX tetramer is composed of two dimers, whose active sites face each other with a slight offset. As a consequence, particular regions of one dimer, such as lid 1 (aa 92-97) and lid 2 (aa 315-322), are situated in the active site of the other dimer. This arrangement of the dimers’ structural lids results in the formation of a channel like cavity and a conformational change in a loop (aa 399-416) situated within the active site. The two lid structures have been greatly associated with increased α-1,4-transferase activity in TreX, and interestingly, their conformation exhibits structural similarity to that of other glucosyltransferase enzymes, which also encompass structural lids close to their active sites. Thus, the implicated function of the lid structures during catalysis is to interact with acceptor molecules, such as glycogen, and provide stability to the complex, so that glycogen may be broken down into long maltooligosacchardies.

Mechanism


In human metabolism, glycogen breakdown involves several enzymes, two of which are glycogen phosphorylase and glycogen-debranching enzyme. Glycogen phosphorylase is responsible for the successive removal of glucose 1-phosphate molecules at the non reducing ends of glycogen branches. However, this enzyme’s activity ceases when it has reached a point four glucose residues away from an (α1→6) branch point. Upon this, the GDE takes over and catalyzes the transfer of three branched glucose units to the nonreducing end of another branch to yield an (α1→4) linkage. The α-1,6-glucosidase activity of the GDE liberates the non-transferred glucose unit involved in a (α1→6) bond.

Although TreX is structurally different from the yeast and mammalian GDE, it does however share functional similarities, and therefore a general mammalian GDE mechanism is shown. In both species the GDE catalyzes an intermolecular transfer  of glucose polymers from one 1,4-α-D-glucan branch to another 1,4-α-D-glucan branch nearby, In addition, both enzymes carry out the hydrolysis of an α-1,6-glycosidic linkage. TreX does however show high specificity for side chains which are composed of 6 or more glucose residues.